1. Field of the Invention
This invention relates to a method of suppressing polymerization of acrylic acid resulting from byproduct contained in an acrylic acid aqueous solution, and precipitation of such byproduct, and more particularly to a method for suppressing polymerization and precipitation in a distillation step of acrylic acid production process, in which part or all of a dischared liquid from the acrylic acid production process is recyled as a relfux, by recycling the discharged liquid as a reflux after permiating the discharged liquid through a reverse osmosis membrane.
2. Description of the Prior Art
Heretofore, a bulky amount of acrylic acid has been produced in a variety of large-scaled plants. A typical production process of acrylic acid comprises the step of reacting propylene and/or acrolein (hereinafter, abbreviated as “propylene”) with a molecular oxygen-containing gas by catalytic gas-phase oxidation to yield reaction gas, the step of contacting the reaction gas with a collecting liquid such as water to yield an acrylic acid aqueous solution, and the step of obtaining crude acrylic acid by distillation of the acrylic acid aqueous solution for purification.
As a result of detailed analysis of the experiments conducted by the inventors of this application, they found that the reaction gas obtained by catalytic gas-phase oxidation of propylene contains, in addition to acrylic acid, unreacted acrolein, and byproducts such as formaldehyde, glyoxal, furfural, benzaldehyde, formic acid, acetic acid, and maleic acid. Consequently, the acrylic acid aqueous solution that has been obtained by contacting the reaction gas with the collecting liquid contains such byproducts mentioned above. It is required to eliminate impurities such as water and byproducts by distillation or its equivalent in order to obtain purified acrylic acid as a final product from the acrylic acid aqueous solution.
Since the relative volatility of acrylic acid to water or acetic acid is small, it is difficult to isolate acrylic acid by simply performing distillation. In view of this, there has been proposed various azeotropic distillation methods for isolating acrylic acid by adding an azeotropic solvent to an acrylic acid aqueous solution for distillation and by extracting a mixture of acetic acid, water, and the azeotropic solvent as distillate.
In such an azeotropic distillation, it is required to set the distillation temperature high in order to remove impurities such as acetic acid whose boiling point is relatively high. Setting the distillation temperature high may likely to cause polymerization of acrylic acid. In view of this, a variety of polymerization inhibitors are fed to the distillation step in order to carry out long-term stable distillation while preventing polymerization of acrylic acid in an attempt to securely prevent polymerization of acrylic acid.
Japanese Unexamined Patent Publication No. HEI 9-95465, for example, discloses a method for preventing polymerization of acrylic acid by introducing copper salt compounds along with N-nitrosophenylhydroxyamine or its salts to a distillation column. Japanese Unexamined Patent Publication No. HEI 6-345681 discloses a method for preventing polymerization of acrylic acid in a distillation column by using 3-component polymerization inhibitor containing N-oxyl compound, phenol compound, and phenothiazine compound in combination with molecular oxygen.
Despite proposal of the various methods, the conventional polymerization inhibitors do not provide sufficient effect in suppressing polymerization of acrylic acid resulting from such byproducts, and precipitation of such byproducts.
Recently, there has been proposed reuse of discharge liquid obtained from the distillation step of isolating acrylic acid, as a collecting liquid which is used to contact with the reaction gas obtained by catalytic gas-phase oxidation of propylene, etc. However, such a discharge liquid contains the aforementioned byproducts. In long-term operation, as the number of times of reusing such a discharge liquid increases, the concentration of the byproducts in the acrylic acid aqueous solution is raised. When acrylic acid is isolated from such an aqueous solution containing the byproducts in high concentration by distillation, it is highly likely that acrylic acid is polymerized due to the byproducts. Adding a polymerization inhibitor in such a high concentrated condition may not effectively suppress the polymerization. Further, such byproducts may be condensed to result in reaction products. Moreover, if such byproducts may polymerize to oligomers, they may likely to precipitate in the distillation column, which resultantly varies the distribution of liquid phase or gas phase in the distillation column and lowers distillation efficiency. Further, the polymerization inhibitor in the distillation column may be distributed unevenly, thus resulting in lowering polymerization suppressive effect or failing to accomplish long-term stable operation of the distillation column.